Delta Force replaced its M4s with the HK416 in 2004, after tests revealed that the piston operating system significantly reduces malfunctions while increasing the life of parts.[4] The HK416 has been tested by the United States military and is in use with some law enforcement agencies and special operations units. It has also been adopted as the standard rifle of the Norwegian Armed Forces and the French Armed Forces.

A modified variant underwent testing by the United States Marine Corps as the M27 Infantry Automatic Rifle. After the Marine Corps Operational Test & Evaluation Activity supervised a round of testing at MCAGCC Twentynine Palms, Fort McCoy, and Camp Shelby (for dust, cold-weather, and hot-weather conditions, respectively). As of March 2012, fielding of 452 IARs has been completed of 4,748 ordered. Five infantry battalions; 1st Light Armored Reconnaissance Battalion and 2nd Battalion, 4th Marines, out of Camp Pendleton, CA, First Battalion, 3rd Marines, out of Marine Corps Base HI, 1st Battalion, 9th Marines, out of Camp Lejeune, NC; and 1st Battalion, 25th Marines, out of Fort Devens, MA have deployed the weapon.[5][6]

The HK416 uses a HK-proprietary short-stroke gas system that derives from the HK G36, forgoing the expanding gas system standard in AR-15 rifles.[7][8] The HK G36 gas system was in turn partially derived from the AR-18 assault rifle designed in 1963.[9] The HK system uses a short-stroke piston driving an operating rod to force the bolt carrier to the rear. This design prevents combustion gases from entering the weapon's interior—a shortcoming with direct impingement systems.[10] The reduction in heat and fouling of the bolt carrier group increases the reliability of the weapon and extends the interval between stoppages. During factory tests the HK416 fired 10,000 rounds in full-auto without malfunctioning.[11] It also reduces operator cleaning time and stress on critical components. According to H&K, "experience that Heckler & Koch gained during its highly successful 'midlife improvement programme' for the British ArmySA80 assault rifle, have now borne fruit in the HK416."[8]

The HK416 is equipped with a proprietary accessory rail forearm with MIL-STD-1913 rails on all four sides. This lets most current accessories for M4/M16-type weapons fit the HK416. The HK416 rail forearm can be installed and removed without tools by using the bolt locking lug as the screwdriver. The rail forearm is "free-floating" and does not contact the barrel, improving accuracy.

The HK416 has an adjustable multi-position telescopic butt stock, offering six different lengths of pull. The shoulder pad can be either convex or concave and the stock features a storage space for maintenance accessories, spare electrical batteries or other small kit items.

The HK416's barrel is cold hammer-forged with a 20,000-round service life and features a 6 grooves 178 mm (7 in) right hand twist. The cold hammer-forging process provides a stronger barrel for greater safety in case of an obstructed bore or for extended firing sessions. Modifications for an over-the-beach (OTB) capability such as drainage holes in the bolt carrier and buffer system are available to let the HK416 fire safely after being submerged in water.

The HK416's outer appearance resembles the M4. It includes international symbols for safe, semi-automatic, and fully automatic. It has a redesigned retractable stock that lets the user rotate the butt plate, and a new pistol grip designed by H&K to more ergonomically fit the hand. A new single-piece hand guard attaches to the rifle with a free floating rail interface system for mounting accessories. The most notable internal difference is the short stroke gas piston system, which is derived from the HK G36. To compensate for increased pressure due to the new gas system, it has a thicker barrel.

Furthermore, an adjustable gas block with piston allows reliable operation on short-barrelled models, with or without a suppressor attached. Finally, the HK416 includes a folding front sight, and a rear sight similar to the G3. The HK416 system is offered as an upper receiver, separate from the rest of the rifle, as a replacement to the standard issue M4 upper receiver. It can attach to existing AR-15 family rifles, giving them the new gas system, the new hand guard, and sights, while retaining the original lower receiver. The Heckler & Koch 416 can also be purchased as a fully assembled, stand alone carbine.

In July 2007, the U.S. Army announced a limited competition between the M4 carbine, FN SCAR, HK416, XCR, and the previously-shelved HK XM8. Ten examples of each of the four competitors were involved. Each weapon fired 6,000 rounds in an extreme dust environment. The shoot-off was for assessing future needs, not to select a replacement for the M4.[12][13] The XM8 scored the best, with only 127 stoppages in 60,000 total rounds, the FN SCAR Light had 226 stoppages, while the HK416 had 233 stoppages. The M4 carbine scored "significantly worse" than the rest of the field with 882 stoppages.[4]

However, magazine failures caused 239 of the M4's 882 failures. Army officials said the new magazines could be combat-ready by spring if testing went well.[14]

The Norwegian Army conducted an extensive evaluation before selecting the HK416 as its new primary firearm.

The Turkish company Makina ve Kimya Endustrisi Kurumu ("Mechanical and Chemical Industry Corporation") has considered manufacturing a copy of the HK416 as the MKEK Mehmetçik-1 for the Turkish Armed Forces.[17] Instead, a new assault rifle has been developed by KALEKALIP with MKEK as the producer, with the Mehmetçik-1 dropped from adoption into the Turkish military.[18][19]

The French armed forces conducted a rifle evaluation and trial, and selected the HK416-F as its primary firearm in 2016.[20]

The HK416 was one of the weapons displayed to U.S. Army officials during an invitation-only Industry Day on November 13, 2008. The goal of the Industry Day was to review current carbine technology prior to writing formal requirements for a future replacement for the M4 carbine.[21][22] The HK416 was then an entry in the Individual Carbine competition to replace the M4. The weapon submitted was known as the HK416A5.[23] It features a stock similar to that of the G28 designated marksman rifle, except slimmer and non-adjustable. The rifle features an improved tool-less adjustable gas regulator for suppressor use, which can accommodate barrel lengths down to 267 mm (10.5 in) without modifications. It also features a redesigned lower receiver with ambidextrous fire controls, optimized magazine and ammunition compatibility, a repair kit housed inside the pistol grip, and a Flat Dark Earth color-scheme.[24] The stock has a fixed buttplate and no longer has a storage space, as well as the sling loops removed from it. The V2 HK Battle grip is incorporated, which has the V2 grip profile with the storage compartment of the V1 grip for tools. The handguard uses a new hexagonal-shaped cross bolt that cannot be removed by the bolt locking lugs, but instead by the takedown tool housed inside the grip.[25] The Individual Carbine competition was cancelled before a winning weapon was chosen.[26] The HK416A5 offers several additional features compared to the preceding HK416 models and has become the standard military and law enforcement model line.[27]

In mid-2014, the German arms procurement agency type classified the HK416A5 with a 16.5 in (420 mm) barrel as the G38 (for Gewehr 38). The carbine version with a 14.5 in (370 mm) barrel received the G38K, another version with an 11 in (280 mm) barrel received the G38C designation in the Bundeswehr catalog (Katalog der Bundeswehr).[28]

HK416C: The ultra-compact variant, with "C" for Commando. The HK416C has a 228 mm (9.0 in) barrel and is expected to produce muzzle velocities of approximately 730 m/s (2,395 ft/s).[29] The firearm's precision is specified as ≈ 4 MOA (12 cm at 100 m) by Heckler & Koch.[29][30][31] The HK416C has a high degree of component commonality with the HK416 family, but uses a shortened buffer tube and a collapsible butt-stock similar to variants of H&K's MP5 sub-machine gun and the HK53 carbine.

HK416A5: Improved carbine entered in the Individual Carbine competition.[24] The competition was cancelled without a weapon chosen.[26]

HK416F: The designated version for the French armed forces. The HK416F replaces the flip-up front sight with a specialized bayonet lug, and uses a custom barrel muzzle to attach and fire rifle grenades.[32]

Civilian variants of the HK416 and HK417 introduced in 2007 were known as MR223 and MR308. Both are semi-automatic rifles with several "sporterized" features. At the 2009 SHOT Show, these two firearms were introduced to the American civilian market renamed respectively MR556 and MR762.[33] There is another variant of the MR556 called the MR556A1, which is an improved version of the former.[34] It was created with input from American special forces units.[35] The MR556A1 lets the upper receiver attach to any M16/M4/AR-15 family lower receiver, as the receiver take-down pins are in the same standard location. The original concept for the MR556 did not allow for this, as the take-down pins were located in a "non-standard" location. The MR223 maintains the "non-standard" location of the pins, disallowing attachment of the upper receiver to the lower receivers of any other M16/M4/AR-15 family of rifles. As of 2012, the MR556A1 upper receiver group fits standard AR-15 lower receivers without modification, and functions reliably with standard STANAG magazines[36]

^Poole, Eric R. (Spring 2012). "Book of the AR-15"(PDF). http://hk-usa.com. Guns and Ammo. p. 27. Retrieved 11 September 2012. In 2006 U.S. Special Operations units in Joint Special Operations Command (JSOC) began fielding the HK416 as their primary carbine.External link in |work= (help)

^Lowe, Christian (April 30, 2007). "Senator Tells Army to Reconsider M4". Military.com. Retrieved 11 September 2012. Special operations forces, including "tier one" units such as the Army's Delta Force and the Navy's SEAL Development Group – or SEAL Team Six – have used their own funds to purchase the Heckler & Koch-built 416, which uses a gas-piston operating system less susceptible to failure than Colt's gas-operated design.

^Cox, Matthew (February 20, 2007). "Better than M4, but you can't have one". marinecorpstimes.com. Military Times. Retrieved 11 September 2012. Members of the elite unit linked up with German arms maker Heckler & Koch, which replaced the M4’s gas system with one that experts say significantly reduces malfunctions while increasing parts life. After exhaustive tests with the help of Delta, the H&K 416 was ready in 2004. Members of the elite commando unit — formally known as 1st Special Forces Operational Detachment-Delta — have been carrying it in combat ever since. … In addition to Delta, experts say the 416 is also in use by other specialized Army units, including the Asymmetric Warfare Group, as well as the Navy’s elite SEAL Team 6.